Boiling water nuclear reactors have been in operation for many years. Commencing with their initial construction and throughout their service lives, these reactors have been known to accumulate debris in their closed circulation moderator (coolant) systems. This debris can become an operating hazard if allowed to enter into the fuel bundle core region containing the heat generating fuel rods. In fact, debris is a leading cause of fuel rod failure in boiling water nuclear reactors (BWR's). In order to understand this problem, a summary of reactor construction as it relates to the accumulation of debris in the core needs will be helpful. Thereafter, the fuel bundle construction will be described with emphasis on the need to preserve substantially unchanged the regions of pressure drop within the fuel bundles. The effects caused by debris entering into the fuel rod region of the fuel bundles will then be summarized.
In boiling water nuclear reactor construction, the reactor is provided with a large, central core. Liquid water coolant/moderator flow enters the core from the bottom and exits the core as a water steam mixture from the top. The core includes many side-by-side fuel bundles, each containing a plurality of fuel rods. Water is introduced into each fuel bundle through a fuel bundle support casting from a high pressure plenum situated below the core. Water passes in a distributed flow through the individual fuel bundles and about the fuel rods where it is heated to generate steam, and then exits the upper portion of the core as a two-phase water steam mixture from which the steam is extracted for the generation of energy.
The core support castings and fuel bundles are a source of pressure loss in the circulation of water through the core. By properly controlling such pressure losses, substantially even distribution of flow across the individual fuel bundles of the reactor core is achieved. When it is remembered that there are as many as 750 individual fuel bundles in a reactor core, it can be appreciated that assurance of the uniformity of flow distribution is important. To interfere with the existing pressure drop within the fuel bundles could negatively affect the overall distribution of coolant/moderator within the fuel bundles of the reactor core.
The fuel bundles for a boiling water nuclear reactor are typically supported between lower and upper tie plate assemblies. The lower tie plate assembly is a one- or two-piece structure including a) an upper grid and 2) a lower inlet nozzle and associated structure providing a transition region from the inlet nozzle to the grid. The inlet nozzle provides for coolant entry to an enlarged flow volume within the flow transition region of the lower tie plate assembly. At the upper end of the flow volume, there is located a tie plate grid. The tie plate grid has two purposes. First, it provides a mechanical support connection for the weight of the individual fuel rods to be transmitted through the lower tie plate assembly to the fuel support casting. Secondly, the tie plate grid provides a path for liquid water moderator to flow into the fuel bundle region for passage between the side-by-side supported fuel rods.
Above the lower tie plate grid, each fuel bundle includes a matrix of upstanding fuel rods, each containing fissionable material which, when undergoing nuclear reaction, transfers energy to the flowing water to produce the power generating steam. The matrix of upstanding fuel rods is engaged at its upper end by the upper tie plate assembly. Usually, water rods also extend (within the fuel rod matrix) between the upper and lower tie plate assemblies for improvement of the water moderator to fuel ratio, particularly in the upper region of the fuel bundle.
Fuel bundles also include a number of fuel rod spacers at varying elevations along the length of each bundle. These spacers are required because the fuel rods are long (about 160 inches) and slender (about 0.4 to 0.5 inches in diameter), and would come into abrading contact under the dynamics of fluid flow and nuclear power generation. The spacers provide appropriate lateral restraints for each fuel rod at their respective elevations and thus prevent abrading contact between the fuel rods and maintain the fuel rods at uniform spacing relative to one another along the length of the fuel bundle for optimum performance. It will be appreciated that these spacers are sites where debris can be trapped and damage the fuel rods.
Each fuel bundle is surrounded by an elongated channel. This channel confines water flowing between the upper and lower tie plate assemblies to a single bundle in an isolated flow path. The channel also serves to separate the steam generating flow path through the fuel bundles from the surrounding core bypass region used for the penetration of the control rods. The water in the bypass region also provides neutron moderation.
In the operation of a boiling water nuclear reactor, maintenance of the originally designed flow distribution is very important. Specifically, from the core inlet to the core outlet, about 20 pounds per square inch (psi) of the pressure drop is encountered at typical flow operating conditions. About 7 to 8 psi of this pressure drop occurs through the inlet orifice and fuel support casting. This pressure drop is mainly to assure the uniform distribution of coolant/moderator flow through the many fuel bundles making up the core of the reactor, and is related to the prevention of operating instabilities within the reactor at certain power rates. At the lower tie plate assembly of each fuel bundle, from the inlet nozzle into the flow volume and through the tie plate grid, about 1 to 11/2 psi pressure drop occurs which contributes to uniform flow distribution between the individual fuel rods of each fuel bundle. Finally, through the fuel bundle itself--from the exit of the lower tie plate assembly to the exit at the upper tie plate assembly--about 11 psi of pressure drop usually occurs. When new fuel bundles are introduced into a reactor core, these flow resistances must be preserved. Otherwise, the coolant/moderator flow distribution could be compromised among the various types of fuel in the reactor core.
With respect to the tie plate grid of the lower tie plate assembly, a matrix of cylindrical bosses and webs generally form the grid. The bosses are sized to receive the fuel rod end plugs. The flow area between the bosses and webs is the primary factor in controlling pressure drop resulting from water flow through the grid.
In early grid constructions, the fuel rods had greater cross-sectional diameters and the bosses were large. In more recent grid constructions, however, the fuel rods have smaller cross-sectional diameters and the bosses are smaller. Also, in early constructions, fewer fuel rods formed a fuel bundle than in recent constructions.
Even with all of these changes in grid and bundle construction, however, it is necessary to avoid significant changes in pressure drop. For example, a core may be composed of older (8.times.8) bundles and newer (10.times.10) bundles, and the flow through each bundle preferably is uniform. One challenge with new fuel bundle constructions, and particularly lower tie plate grid constructions, is to accommodate more fuel rods and to perform a debris catching function, yet maintain a flow rate substantially equivalent to the flow experienced in older bundle constructions.
Typically, debris within boiling water nuclear reactors can include extraneous materials left over from reactor construction, and outage maintenance and repair activities. During the numerous outages and repairs, even further debris accumulates. Because nuclear reactors constitute closed circulation systems, it will be appreciated that debris will essentially accumulate with increasing age and use of the reactor. A particularly vexing but usual place for the accumulation of debris is in the fuel bundles between the fuel rods, and particularly in the vicinity of the fuel rod spacers. Debris particles tend to lodge between the spacer structure and the fuel rods and often dynamically vibrate with the coolant/moderator flow in abrading contact to the sealed cladding of the fuel rods.